Expression of bone markers and micro-CT analysis of alveolar bone during orthodontic relapse.

OBJECTIVES: To investigate biological changes in alveolar bone occurring during orthodontic relapse. MATERIALS AND METHODS: Rat maxillary first molars were moved mesially for 10 days. After orthodontic tooth movement (OTM), appliances were removed, and the molars were allowed to relapse for one, three, five, seven, 14 or 21 days. Changes in 3D morphometric parameters of bone located mesial to the first molars were evaluated by micro-CT. Total RNA was isolated from the same bone site, and real-time RT-PCR was used to measure the expression of bone formation and resorption markers. RESULTS: One day after appliance removal, the molars relapsed to a mean 73% of the achieved OTM and then steadily relapsed to 93% at 21 days. Tissue mineral density and per cent bone volume increased over the experimental period. Inversely, there was a decrease in total porosity. Gene expression of OCN, Coll-I and ALP decreased during OTM, whilst as the molars relapsed showed tended to increase. Gene expression of RANKL and TRAP increased during OTM. Changes in mRNA expression of H(+)-ATPase were minor. By 21 days post-appliance removal, the remodelling process in rats appeared to have returned to control levels. CONCLUSIONS: Bone tissue reactions on a molecular level are similar during OTM and orthodontic relapse. These findings validate the importance of immediate retention following active OTM.